26 



STOMATA. 



53. Stomata occur on the green parts of plants, especially on the 

 leaves and their appendages. They are not usually found in cel- 

 lular plants, nor in plants always submerged, nor in pale parasites. 



74 75 



This is not, however, a universal rule, for stomata have been de- 

 tected in Marchantia and some other Cellulares; also in the sub- 

 merged leaves of Eriocaulon setaceum, according to Griffith, and in the 

 pale parasite Orobanche Eryngii, according to Duchartre. They do 

 not exist in roots, nor in plants kept long in darkness so as to be 

 blanched or etiolated, and they are rare or imperfectly developed in 

 succulent plants. 



54. Stomata vary in their form. 



* The oval form is very common, 



and may be easily seen in Lili- 

 aceous plants ; the spherical oc- 

 curs in Oncidium altissimum and 

 the Primrose, the quadrangular in 

 Yucca and Agave. In the Ole- 

 ander, in place of stomata there 

 are cavities in the epidermis pro- 

 tected by hairs (fig. 76 s). 

 55. The development of stomata has been traced by Mirbel and 

 Mohl. In the Hyacinthus orientalis, they appear first between the 

 epidermal cells in the form of quadrangular spaces containing gra- 

 nular matter, which gradually collects towards the centre, where a 

 septum or partition is formed. This septum ultimately splits, leaving 

 a slit or opening which constitutes the stoma. Mohl has traced this 



Fig. 74. Vertical section of epidermis, from the lower surface of the leaf of Madder, show-ing 

 the intimate union of the epidermal cells, e e, the loose subjacent parenchyma, p, with intercel- 

 lular canals, TO, and lacuna, 1. s, Stoma. 



Fig. 75. Epidermis of leaf of Saxifraga sarmentosa, showing clusters of stomata, s s, surrounded 

 by large epidermal cells, e e. The cells among which the stomata occur are very small. 



Fig. 76. Vertical section of lower epidermis of the leaf of A'erium Oleander, e, Epidermis 

 composed of several layers of cells, p. Parenchyma of the leaf. , Cavity filled with hairs, which 

 may represent a stoma. 



